1. Field of the Invention
This invention relates to methods for coating implantable medical devices, such as stents.
2. Description of the Background
FIG. 1 illustrates a conventional stent 10, which includes connected struts 12 forming a tubular expandable body. Stent 10 functions as a scaffolding structure for physically holding open the wall of a blood vessel or other bodily lumen. Stent 10 is capable of being compressed, so that stent 10 can be inserted through small lumens via catheters, and then expanded to a larger diameter once it is at the desired location. Mechanical intervention via stents has reduced the rate of restenosis as compared to balloon angioplasty; restenosis, however, is still a significant problem. Moreover, treating restenosis in stented vessels can be challenging, as clinical options are more limited as compared to lesions that were treated solely with a balloon.
In order to more effectively treat restenosis, stent implantation procedures are being supplemented with a pharmaceutical regimen. Systemic administration of drugs for the treatment of restenosis can produce adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
Being made of metal, stents need to be modified so as to provide a suitable means of locally delivering a drug. A polymeric coated stent has proved to be a very effective way of allowing a stent to locally deliver a drug. A solution of a polymer dissolved in a solvent and a therapeutic substance added thereto is applied to the stent. The composition is applied to the stent by spraying the composition on the stent or immersing the stent in the composition. Once the solvent evaporates, a polymeric coating impregnated with a therapeutic substance remains on the surface of the stent. The coating provides for a sustained release of the therapeutic substance at the treatment site.
To the extent that the mechanical functionality of stents has been optimized, continued improvements can be made to the coating of the stent. A coating design is needed that is capable of releasing more than one therapeutic substance to the treatment site. Accordingly, conditions other than restenosis, such as excessive inflammation or thrombosis, can also be addressed. Moreover, the coating should be capable of releasing a single drug or more than one drug at different release rates. For example, a coating should be capable of releasing a steroidal anti-inflammatory substance immediately subsequent to the stent implantation and releasing a drug for inhibiting migration and proliferation of vascular smooth muscle cells at a slower release rate for a prolonged duration of time. Accordingly, a more customized treatment regimen for the patient can be provided. The present invention provides an apparatus that can produce a coating that addresses these needs and provides other improved coating designs for drug eluting vascular stents.